Silicon carbide is a material characterized by manifesting high thermal conductivity, generally excelling in heat resistance and mechanical strength, exhibiting physical and chemical stability evinced by strong durability to withstand radioactive rays, and enjoying a broad energy bandgap. Particularly, the 6H type silicon carbide crystal possesses a forbidden bandwidth of about 3 eV at room temperature and therefore secures adoption as a blue light-emitting diode. Thus, it can be utilized as materials for environmental resistance devices, radioactive ray resistance devices, electric power controlling devices and short-wavelength light-emitting devices that are usable at elevated temperatures.
As a means to produce the silicon carbide single crystal, the method of sublimation using silicon carbide powder as a raw material at an elevated temperature is generally used (International Unexamined Patent Publication HEI 3-501118, for example).
In the production of a silicon carbide single crystal by the method of sublimation, a crucible packed with silicon carbide powder as a raw material and having a seed crystal substrate mounted therein is decompressed in an atmosphere of an inert gas and the entire apparatus is heated to a temperature in the range of 1800 to 2400° C. The chemical seed (gas) generated by the decomposition and sublimation of the silicon carbide powder in consequence of the heating reaches the surface of the seed crystal substrate retained in the range of growth temperature and undergoes epitaxial growth as a seed crystal. Wafers of such seed crystal grown to the approximate level of 3 inches are now available on the market.
In consequence of the elevation of temperature, the silicon carbide as the raw material emits the vapors of Si, Si2C, SiC2, SiC, etc. which contribute to the growth of crystal and induces floatation in the crucible of fine particles of impurities and crystallinity-interfering fine particles which are entrained by the raw material as well. It is said that the adhesion of such fine particles of impurities to the surface of the single crystal growing from the seed crystal substrate disposed opposite the layer of silicon carbide raw material in the crucible forms the cause for generation of micro-pipes in the epitaxially growing single crystal and occurrence of dislocation of the crystal.
For the sake of manufacturing a single crystal of silicon carbide into a seed crystal substrate, it is subjected to formation working that comprises as grinding, washing and chemical treatment. The seed crystal substrate retains on the surface thereof disturbances, such as a degenerated layer induced during the course of the formation working. Since the denatured layer retained on the silicon carbide is chemically stable, it is not easily removed owing to the unavailability of a proper etchant. In the ordinary method of sublimation, therefore, the crystal defect in the surface of the seed crystal substrate forms the cause for frequent occurrence of crystal defects, such as micro-pipes and helical crystal dislocation. Further, the conventional method of sublimation has encountered difficulty in controlling the shape of crystal and the surface of crystal because it induces growth of a crystal by spontaneous formation of a nucleus.
The conventional method of sublimation has been unable to preclude occurrence of numerous kinds of crystal defects, such as micro-pipes, helical crystal dislocation and small inclination grain boundary in the grown single crystal owing to the improper selection of the crystal faces and the growth conditions. Thus, the silicon carbide wafers currently manufactured and marketed suffer the occurrence of numerous defects that result in imparting a degraded quality to the electric devices using substrates of silicon carbide.
In the case of a Si bulk crystal, for example, the problem of such intrinsic defects as exist in a seed crystal is solved through adoption of a procedure that comprises tentatively thinning the seed crystal by means of the so-called necking treatment, thereby reducing the defects, and then enlarging the diameter to consequently obtain a substrate of a large diameter. Also in the case of silicon carbide, for the purpose of removing the causes for crystal defects one by one, a concept of reducing defects by squeezing a given crystal tentatively to a thin diameter and then allowing it to grow has been proposed (JP-A HEI 5-319998, for example).
In the case of silicon carbide, however, this method cannot thoroughly reduce crystal defects and cannot preclude the persistence of dislocations and defects of the extents of 5×104 to 3×105/cm2 even after the necking treatment. Further, a concept of reducing micro-pipes by deliberately mingling impurities in the single crystal in a ratio of 1019/cm2 or more has been proposed (JP-A HEI 9-157092, for example). The impurities so mingled tend to form nuclei of crystalline polymorph different in crystal structure from the single crystal of silicon carbide and induce frequent incorporation of crystalline polymorph.
This invention, in attaining crystal growth using a seed crystal of a large diameter formed by the method of sublimation and entailing defects ascribable thereto, is aimed at developing a method for producing a high-quality silicon carbide single crystal by making the growth of crystal without inducing new defects during the course of crystal growth.